1. Field of the Invention
The present invention relates to an optical polarimeter for measuring the space between a transparent member and a reflective member.
2. Description of Related Art
Hard disk drives contain a magnetic transducer(s) that magnetizes a flat rotating disk. The magnetic transducer is typically assembled to a suspension arm as part of a head gimbal assembly (HGA) that is suspended from an actuator arm. The actuator arm has a motor that moves the slider across the disk surface. Rotation of the disk creates an airstream which lifts the slider off of the disk surface. The air bearing created by the rotating disk prevents the slider and disk material from structurally wearing. The thickness of the air bearing can vary depending upon the spring rate of the suspension arm, aerodynamic characteristics of the slider and other factors.
When mass producing hard disk drives, it is desirable to measure the air bearing for each HGA to insure that the air bearing thickness is within operational tolerances. Various optical systems have been developed to detect the microinch and submicroinch air bearing thicknesses typically created in hard disk drive units. The air bearing thickness can be measured in optical systems by inserting the suspension arm into a test unit which has a transparent glass substrate. A light beam is then directed through the glass substrate and onto the slider. The reflection of light from the slider and the substrate air bearing interface creates an interference pattern that is detected by a photodetector. The thickness of the air bearing is computed from the interference pattern. The light source and photodetector are typically at approximately normal incidence to the slider and transparent substrate.
Interferometric testers typically require a calibration procedure to determine the maxima and minima of the interference pattern. The maxima and minima may be determined by varying the air bearing thicknesses, either by changing the rotational speed of the disk, or by mechanically unloading the slider. Once the maxima and minima are known, the air bearing thickness associated with a given detector signal can be calculated from the known functional form of the multiple-beam interference.
It has been found that some slider designs do not produce a wide enough range of air bearing thicknesses in response to variations in disk speed to determine maxima and minima. Retraction by mechanically unloading the slider has also been found to introduce errors because of slider tilt. In addition, the rate of change of light intensity goes to zero when the air bearing thickness is varied through an interference minimum or maximum. Additionally, it has been found that the signal to noise ratio degrades as the slider approaches the minimum associated with slider to disk contact.
For these reasons, prior art interferometers, such as the Phase Metrics DFHT, incorporate multiple wavelengths, and corresponding multiple detectors for signal to noise ratio improvements and for coping with loss of flying height measurement sensitivity at spacings which correspond to interferometric maxima and minima. A second type of multiple wavelength tester was marketed by IBM under the trademark CRAMA. The CRAMA system uses a two dimensional detector array in combination with a least square fit computation process to recover phase and determine air bearing thickness at each pixel of the interferogram image.
U.S. Pat. No. 5,218,424 issued to Sommargren, discloses an interferometer that functions at an incident angle that is not normal to the substrate. The Sommargren tester generates two laterally displaced perpendicularly polarized coherent light beams that are directed through the glass substrate at Brewster's angle for the substrate-air interface. One of the polarized light beams is transmitted through the substrate and reflected off of the slider. The other beam is reflected from the glass-air bearing interface. The reflected polarized light beams are recombined and directed onto a two-dimensional photodetector array. The Sommargren system includes a phase shifter that shifts the phase of the polarized beams directed onto the substrate. The detector measures the relative phase difference between the light reflected from the slider and the substrate-air bearing interface. The air bearing thickness is proportional to the phase difference of the light beams.
The Sommargren technique is advantageous because a calibration technique is not required to detect the maxima and minima for an interference signal. Additionally, intensity sensitivity is uniform and does not approach zero at the maximum and minimum locations. Unfortunately the Sommargren system will not compensate for variations in the real index of refraction n and extinction coefficient k of the slider. These coefficients may vary from slider to slider and create inaccurate measurements. Additionally, the Sommargren technique is limited to operation at Brewster's angle. Finally, the Sommargren tester does not make an absolute measurement of spacing but requires some independent calibration to find a zero air gap. It would be desirable to provide a tester that can make an absolute spacing measurement, was not restricted to a specific angle and compensated for the varying optical properties of different sliders.